1. Field of the Invention
The present invention relates to an optical communication system utilizing signal light including respective wavelength components assigned to a plurality of channels; and, in particular, to an optical communication system in which Raman amplification compensates for the transmission loss occurring when the signal light propagates through an optical transmission line.
2. Related Background Art
In an optical communication system utilizing signal light (wavelength division multiplexing light) including respective wavelength components assigned to a plurality of channels within a signal wavelength band, the signal light transmitted from a transmitter incurs transmission loss while propagating through the optical transmission line, so that the signal light decreases its power when reaching a receiver. If the signal light having reached the receiver has a power not higher than a predetermined value, reception errors may occur, whereby normal optical communications may not be carried out. Therefore, an optical amplifier is disposed between the transmitter and receiver, and the signal light is amplified by the optical amplifier, whereby the transmission loss occurring in the signal light propagating through the optical transmission line is compensated for.
Known as such an optical amplifier are optical fiber amplifiers doped with rare-earth elements (e.g., Er-doped optical fiber amplifier) and Raman amplifiers utilizing Raman amplification phenomena in Raman amplification optical fibers. As compared with the optical fibers doped with rare-earth elements, the Raman amplifiers have such characteristics that they can adjust a gain-yielding wavelength band by appropriately setting the wavelength of Raman amplification pumping light, and the like.
In wavelength division multiplexing (WDM) optical communication systems for carrying out optical communications by utilizing signal light in which a plurality of wavelength components within a predetermined signal wavelength band are multiplexed, it is important that optical amplifiers in this signal wavelength band have a flat gain spectrum; otherwise, even when a signal of a signal channel within the signal wavelength band is correctly received by the receiver, signals of other signal channels having a lower gain may yield reception errors. Hence, studies have been conducted concerning techniques for flattening the gain spectrum of Raman amplifiers.
For example, in the gain flattening technique for Raman amplifiers disclosed in literature 1, i.e., Y. Emori, et al., xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes, xe2x80x9d OFC""99, PD19 (1999), respective output light components from N (Nxe2x89xa72) pumping light sources are multiplexed, and thus multiplexed light is supplied as Raman amplification pumping light to a Raman amplification optical fiber. Also, the output center wavelength and output power of each of the N pumping light sources are appropriately set, so as to flatten the gain spectrum of Raman amplifiers. In literature 1, the number of pumping light sources, N, is 12.
In the gain flattening technique for a Raman amplifier disclosed in literature 2, i.e., F. Koch, et al., xe2x80x9cBroadband gain flattened Raman amplifier to extend operation in the third telecommunication window, xe2x80x9d OFC""2000, ThD, FF3 (2000), the gain spectrum of Raman amplifier is flattened by a gain equalizer having a loss spectrum with a shape substantially identical to that of the gain spectrum in the Raman amplification optical fiber.
The inventors studied conventional optical communication systems and, as a result, have found problems as follows. Namely, there is a case where M (Mxe2x89xa72) Raman amplifiers are necessary between a transmitter and a receiver in an optical communication system for carrying out long-distance optical communications. If the above-mentioned gain flattening technique in conventional Raman amplifiers (literature 1) is employed in this case, the total number of pumping light sources necessary in the whole optical communication system will be Mxc3x97N, thereby raising the manufacturing cost.
In Raman amplifiers employing the gain flattening technique disclosed in literature 2, the pumping efficiency is low due to its structure in which, while the signal light is Raman-amplified by Raman amplification optical fibers, thus amplified signal light is attenuated by gain equalizers and the like.
For overcoming the problems mentioned above, it is an object of the present invention to provide an optical communication system having a flat Raman gain spectrum and excellent pumping efficiency in a signal wavelength band, and comprising a structure which is realizable/operable at a low cost.
The optical communication system according to the present invention is an optical communication system which transmits signal light (WDM light) including respective wavelength components assigned to a plurality of signal channels within a signal wavelength band from a first point to a second point. This optical communication system comprises an optical transmission line including a plurality of Raman amplification optical fibers, and respective pumping light suppliers provided so as to correspond to the Raman amplification optical fibers.
The optical transmission line transmits signal light from the first point to the second point, whereas each of a plurality of Raman amplification optical fibers constituting at least a part of the optical transmission line Raman-amplifies the signal light when the Raman amplification pumping light is supplied thereto. Each of the pumping light suppliers supplies Raman amplification pumping light to its corresponding Raman amplification optical fiber.
In the optical communication system according to the present invention, in particular, two Raman amplification optical fibers selected from the plurality of Raman amplification optical fibers included in the optical transmission line differ from each other in one of the wavelength at which the gain of Raman amplification becomes the highest and the number of signal channels at which the gain of Raman amplification is maximum. Also, in the optical communication system, the pumping light suppliers provided so as to correspond to the selected two Raman amplification optical fibers may differ from each other in the number of pumping light sources contained therein. Preferably, in the optical communication system, a value obtained by integrating the absolute value of difference between respective gain spectra of Raman amplification in the selected two Raman amplification optical fibers with respect to wavelength is at least 7.5 dBxc2x7nm.
In the optical communication system according to the present invention, signal light including a plurality of wavelength components (assigned to respective signal channels) within a signal wavelength band sent out from a first point (transmitter or repeater) propagates through an optical transmission line including a plurality of Raman amplification optical fibers, so as to reach a second point (receiver or repeater). Here, the optical transmission line may include an optical fiber laid in a repeating section, and an optical fiber within a repeater, one of which is a Raman amplification optical fiber. Each of the plurality of Raman amplification optical fibers receives Raman amplification pumping light supplied from its corresponding pumping light supplier, transmits signal light, and Raman-amplifies the signal light. In the optical communication system according to the present invention, in particular, two selected Raman amplification optical fibers are different from each other in one of the wavelength at which the gain of Raman amplification becomes the highest and the number of signal channels at which the gain of Raman amplification is maximum. Preferably, they are further different from each other in the number of pumping light sources included in the respective pumping light suppliers provided so as to correspond thereto. In such a configuration, though the gain spectrum of Raman amplification in each Raman amplification optical fiber does not become flat within the signal wavelength band, at least the gain spectrum of Raman amplification viewed from the whole optical transmission line from the first point to the second point can be flattened within the signal wavelength band. The optical communication system according to the present invention can reduce the total number of pumping light sources necessary in the whole system as compared with the system disclosed in literature 1. Also, this can greatly lower the manufacturing and operating cost of the system. Further, since this optical communication system does not have a structure in which signal light is attenuated by a gain equalizer so as to flatten the gain, a better pumping efficiency is obtained as compared with the system disclosed in literature 2.